i.MX6SX has a single LVDS port and share a similar LDB_CTRL register layout
with i.MX8MP and i.MX93.

There is no LVDS CTRL register on the i.MX6SX, so only write to
this register on the appropriate SoCs.

Add support for the i.MX6SX LDB.

Tested on a imx6sx-sdb board with a Hannstar HSD100PXN1 LVDS panel
and also on a custom i.MX6SX-based board.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Marek Vasut <marex@denx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20230404020129.509356-2-festevam@gmail.com